1. Field of the Invention
The present intention relates to a motor driving method and a motor driving apparatus for driving a plurality of motors to which electrical power is supplied from a common power supply. More particularly, the present invention relates to a motor driving method and a motor driving apparatus for driving motors in which one of the motors is a brushless motor that is driven by a direct PWM driving method, which can securely prevent problems caused by a reverse regeneration current generated in the brushless motor.
2. Related Art
An apparatus may use a plurality of motors. For example, a video recording/reproducing apparatus is equipped with a brushless motor as a capstan motor, a motor with brush for loading and unloading recording media, and a brushless motor for driving a cylinder head. Among the motors, the brushless motor as a capstan motor is generally driven by a direct pulse width modulation (PWM) driving method to reduce the driving power consumption.
In the direct PWM driving method, a power transistor that switches current conducting through a driving coil is turned on and off, for example, by a bi-directional current conduction method, in a cycle shorter than a current conduction switching cycle, such that a regeneration current is flown by a back electromotive force generated by the driving coil during an off period when an external current is shut off, to thereby maintain a driving torque. By driving the brushless motor by the direct PWM driving method, an external current supply is not required while the driving coil generates the regeneration current. As a result, the method is very effective in reducing the energy consumption for driving the motor.
Referring to FIGS. 2 through 4, a conventional direct PWM driving method for driving a brushless motor is described below.
FIGS. 2(a) and 2(b) show a coil current waveform and a coil voltage waveform for one phase in the direct PWM driving method for driving a three-phase brushless motor, respectively. FIGS. 3(a) and 3(b) show the coil current waveform and the coil voltage waveform in detail (for section a shown in FIGS. 2(a) and 2(b)) when a direct PWM current is applied.
Section b in FIGS. 3(a) and 3(b) represents a state in which a power transistor for switching conduction of the drive coil current is turned on, which corresponds to a period during which current is supplied from the motor power supply VM. During this period, the current of the one-phase driving coil increases as indicated by the current waveform in FIG. 3(a) according to the time constant of the coil. In contrast, section c in FIGS. 3(a) and 3(b) represents a state in which the power transistor for switching conduction of the drive coil current is turned off, which corresponds to a period during which the driving coil itself generates a back electromotive force and flows a regeneration current. During this period, the current of the one-phase driving coil decreases according to the time constant of the coil.
FIG. 4(a) shows a motor current (IMb) path that is created when the power transistor for switching conduction of the drive coil current is turned on during section b in FIGS. 3(a) and 3(b). In this state, the motor current flows through a power transistor Q1 on the side of the power supply voltage (VM) to the driving coils Lu and Lv, and returns through a power transistor Q4 on the side of the motor grounding (M-GND) to the motor power supply. The power transistor Q4 on the side of the motor grounding is maintained at an ON state until the conducting current switches to the other phase. On the other hand, the power transistor Q1 on the power supply voltage side is repeatedly turned on and off as shown in FIG. 3.
FIG. 4(b) shows a motor current (IMc) path during section c shown in FIGS. 3(a) and 3(b). In this case, as the power transistor Q1 is turned off, the driving coils generate back electromotive forces E1 and E2, respectively. As a result, a regeneration current IMC flows from the power transistor Q4 through the motor grounding (M-GMD) line to a diode D3 associated with the other power transistor Q3 on the grounding-side. This regeneration current is a motor current that flows through the driving coils.
In the direct PWM driving method described above, a part of the motor current is supplemented by a regeneration current that is generated by the motor itself As a result, the current that may have to be externally supplied can be reduced.
As described above, the motor driving method using the conventional direct PWM driving method is effective in reducing the energy consumption in driving the motor. However, there is a possibility in which the regeneration current does not flow within the driving circuit, but may flow in a reverse direction to the side of the motor power supply (VM).
For example, when the current conducting through the driving coil is switched during the timing shown in FIG. 4(b), and the power transistor Q4 is turned off, a regeneration current IMcxe2x80x2 would likely flow in a path indicated in FIG. 5(a) due to back electromotive forces E1xe2x80x2 and E2xe2x80x2 generated on the respective driving coils. As this moment, if the sinking capability on the side of the motor power supply VM is insufficient, the regeneration current IMcxe2x80x2 flowing in a reverse direction cannot go anywhere, such that the motor power supply VM is rapidly elevated as the back electromotive forces E1xe2x80x2 and E2xe2x80x2 pull up the motor power supply VM. As the motor power supply increases and exceeds the breakdown voltage of the motor driving circuit, the driving circuit would be destroyed. In particular, when the brushless motor is in the reverse rotation brake mode, the PWM off timing shown in FIG. 5(a) becomes longer. As a result, the motor power supply VM would likely elevate, and therefore a dielectric breakdown of the driving circuit would likely occur.
Accordingly, as shown in FIG. 5(b), for example, an electrolytic capacitor C of 100 xcexcF or greater, a Zener diode Dz of a high current capacity or the like may be added between the motor power supply line L (VM) and the motor grounding line L (M-GND), to absorb the regeneration current IMcxe2x80x2 to protect the driving circuit from a dielectric breakdown. However, the addition of such a protection circuit leads to a higher cost.
It is an object of the present invention to provide a motor driving method and a motor driving apparatus that can avoid problems arising from a reverse regeneration current without adding a protection circuit.
To solve the problems described above, in accordance with one embodiment of the present invention, a method is provided for driving a motor apparatus having a first motor, a second motor, a motor power supply commonly used for the first motor and the second motor, wherein the first motor is a brushless motor. In one aspect of the present invention, an electrical power is supplied from the motor power supply to driving circuits of the first motor and the second motor through a common motor power supply line; the first motor is driven by a direct PWM driving method; the second motor is driven by a method different from the direct PWM driving method; and a reverse regeneration current generated in the first motor is flown to the second motor through the common motor power supply line.
In accordance with the present invention, for example, when a brushless motor of a type that is driven by a direct PWM driving method and a motor of a type that is driven by a normal driving method are present in one system, such as, for example, a video recording and reproducing apparatus, a motor power supply line is commonly used by both of the motors, and a reverse regeneration current generated in the brushless motor driven by the direct PWM driving method is flown in the other motor driven by the normal driving method through the common motor power supply line, such that the reverse regeneration current is absorbed by the motor driven by the normal driving method. As a result, a dielectric breakdown of the motor driving circuit can be prevented.
When the brushless motor driven by the direct PWM driving method, i.e., the first motor undergoes a shift to a reverse brake mode when the second motor is stopped, the second motor that is driven by the normal driving method may preferably be started in synchronism with the shift to the reverse brake mode.
Also, when the brushless motor driven by the direct PWM driving method undergoes the shift to the reverse brake mode while the second motor is operated, the second motor that is driven by the normal driving method is temporarily accelerated in synchronism with the shift to the reverse brake mode.
In the reverse rotation brake mode, a rapid increase in the motor power supply voltage would likely occur. Accordingly, by starting or accelerating the second motor when the first motor is in the reverse rotation brake mode, a reverse regeneration current generated can be effectively consumed as a part of the driving current for the second motor, whereby the reverse regeneration current can be securely absorbed.
In accordance with another embodiment of the present invention, a motor driving apparatus drives a motor by the motor driving method described above. In one aspect of the present invention, a first motor driving circuit that drives the first motor by the direct PWM driving method and a second motor driving circuit that drives the second motor are formed in a common IC chip.
By implementing both of the driving circuits in a common IC chip, their wiring resistance loss is substantially lowered compared to a case in which the driving circuits are formed as discrete circuits on independent substrates. Accordingly, a reverse regeneration current generated can be effectively absorbed on the side of the second motor that is driven by the normal driving method. As a result, an elevation in the driving voltage on the motor power supply line can be suppressed to a minimum value.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.